The present invention relates to a method of making PVD-coated cutting tools comprising the deposition of a PVD coating wherein the coating, during the deposition, is subjected to more than one ion etching step. Cutting tools made according to the present invention exhibit an increased tool life due to increased smoothness of the PVD coating which is itself due to a reduced number of surface defects.
Most cutting tools today are coated with PVD or CVD coatings like Ti(C,N), TiN, (Ti,Al)N, (Ti,Si)N, (Al,Cr)N or Al2O3. PVD coatings are generally thinner than CVD coatings and are often used in applications where toughness is essential, like milling. PVD coatings have several attractive properties compared to CVD-coatings, for instance, greater compressive stress, finer grained coatings and a better ability to tolerate changes in load. Unfortunately, most arc-deposited PVD-coatings suffer from the existence of metallic macroparticles, also called macros or droplets, which exist as small spheres on the surface of the coating or buried inside the coating. During the deposition of the coating, these macroparticles can shadow the incoming flux of charged metal ions, thus creating voids in the coating in the immediate surroundings of the macroparticle. Due to the subsequent diminished adhesion between the macroparticle and the coating, the macroparticle can fall out either during the deposition process or immediately afterwards or even during machining. This may result in an inferior coating quality, with voids, pores or even, in extreme cases, holes straight down to the substrate.
When a coating having macroparticles is used, for example, in a milling operation, there is a risk that the chip will, mechanically and/or chemically, interact with a protruding macroparticle. As the chip flows over the cutting tool surface a small part of the coating may be torn off, leaving a hole or a pore in the coating. This pore or void can be the starting point of cracks. The existence of cracks on the cutting edge will eventually lead to edge chipping or cutting tool breakage.
Ion etching is a common step in the beginning of all kinds of deposition processes. The substrate is usually ion etched prior to deposition to remove surface contaminants and native oxides and nitrides.
U.S. Pat. No. 6,033,734 describe different etching processes performed on a metal substrate surface prior to deposition of PVD coatings. The etching is a metal etching process which is performed by using Cr and/or Mo ions.
However, there have been a few attempts to use ion etching as an intermediate step.
EP 0756019 describes a method of making a PVD-coating for material deforming tools used in punching operations. A PVD-layer composed of (Ti,Al)N, (Ti,Al,Y)N or (Ti,Al,Cr)N or any multilayer thereof is deposited. The surface is then mechanically treated e.g. with sand blasting or metal ion etching to remove any droplets and to achieve a smooth surface. A second, low-friction, PVD coating consisting of MoS2 is then deposited on top.
EP 1 533 210 A describes a process for depositing PVD α-Al2O3. The substrate is first coated with a first hard coating which can be subjected to an ion bombardment either with argon or metal ions. The bombarded surface is then subjected to an oxidation process whereby an oxide surface layer is formed. Onto this oxide surface layer an Al2O3 layer is deposited which will be α-Al2O3.